The use of endoprostheses to eliminate pain and restore the function of the hip joint, especially in patients suffering from osteoarthritis, is a technique that was developed starting from the second half of the last century and is by now well established in the medical art. In most cases, where both the acetabular cavity and the femoral head of the patient are compromised by the pathology, a total arthroprosthesis of the hip, also called THR—acronym for total hip replacement—is performed. This operating technique entails the implantation of an acetabular prosthesis in the patient's pelvis on the one hand and, on the other hand, the resection of the femoral head and replacement thereof with a prosthetic stem endowed with an articulating ball that fits into the aforesaid prosthesis.
Despite having led to some excellent clinical results in terms of functional recovery of the joint, arthroprosthesis of the hip has some major drawbacks. In particular, it has been observed, due to the reduced diameter of the prosthetic ball, there are frequent episodes of displacement of the artificial joint; not rarely, for the same reason, an arthroprosthesis performed without the necessary precision and expertise may lead to a disparity in the length of the lower limbs. Finally, arthroprosthesis of the hip does not enable the patient to engage in sports or other stressful activities, making this an unsuitable solution especially for younger patients.
In order to remedy the drawbacks observed in the art, an alternative to the traditional arthroprosthesis of the hip has been developed in recent times, namely, hip resurfacing. This approach involves capping the articular surface of the pelvis and femoral head with metal cups of modest thickness, thereby preserving both the head and neck of the patient's femur. This operation makes it possible to maintain a joint diameter that is close to the physiological one, thus reducing the risk of displacements and modifications in the length of the limb. Furthermore, the wear on the components is more limited, with a consequent lengthening in the average lifespan of the prosthesis.
Both operating approaches summarily described here have in common the need to prepare the surface of the acetabulum and implant therein an acetabular prosthesis. Preparation entails first of all a step of cleaning the bone, removing in particular the soft tissues and cartilage which preclude correct visualization of the bony landmarks (e.g. the acetabular fossa) present on the periphery of the acetabulum. So-called acetabular reaming is then performed, using a special tool inserted in the cavity. Acetabular reaming serves to smooth the internal surface of the cavity in order to prepare it to receive the acetabular prosthesis.
Another tool, called an impactor, is used to position and fix the acetabular prosthesis. This tool has a stem, to the end of which the replacement cotyle is fixed; it enables the prosthetic device both to be inserted in the bone cavity and mechanically locked in place before the stem is withdrawn.
The above-mentioned operations of preparing, positioning and fixing the acetabular prosthesis within the corresponding pelvic cavity are extremely critical and delicate steps in an arthroplasty procedure. The orientation of the acetabular prosthesis, which is defined by these operations, in fact largely determines the implant's success in the long term. A correct positioning will result in an optimal distribution of loads and an ideal stability of the prosthesis; an incorrect positioning, on the contrary, may result in a rapid deterioration of the implant or in biological complications of another kind.
It should be observed, moreover, that the aforesaid procedures are complicated by the difficult access to the bone site, and by the limited visibility of the latter. In particular, while in the hip arthroprosthesis operations the removal of the femoral neck serves in some way to make the route toward the acetabulum more pervious, in hip resurfacing arthroplasty the surgeon does not enjoy this relative advantage.
It may further be observed that the use of specific guides realized with rapid prototyping systems, currently applied in other orthopaedic surgical procedures, cannot be easily transferred to operations on the acetabulum. In fact, a vast, easily exposable bone area ensuring a stable positioning on the physical guide is lacking around the acetabular cavity.
International patent WO 2011/117644 proposes a set of instruments that the surgeon can use to perform operations on the acetabulum, comprising: locating means, temporarily located in the acetabulum; guide means, set in a pre-established position in relation to the locating means and serving as a guide, when in that position, for the instruments used to prepare of the acetabular surface (acetabular reamer) and position and fix the prosthesis (impactor); as well as support means for maintaining the guide means in that position when the locating means have been removed.